Patterns constituting a large-scale integrated circuit (LSI), as exemplified by DRAM of a gigabit class, have a minimum feature size on the order of submicron to nanometer. One of major causes for yield reduction in a manufacturing process of such an LSI includes defects on a reticle (also called a mask) used when a fine pattern is exposed and formed onto a semiconductor wafer using lithography technology.
Particularly with increasingly finer pattern dimensions of LSI formed on a semiconductor wafer, dimensions that must be detected as pattern defects are also becoming extremely smaller. Thus, apparatuses for inspecting for extremely small defects are vigorously being developed.
With progression of multimedia, on the other hand, an LCD is getting increasingly larger with a liquid crystal substrate size of 500 mm×600 mm or more and a pattern such as a thin-film transistor (TFT) formed on the liquid crystal is becoming increasingly finer, demanding an extensive inspection of extremely small pattern defects. Thus, development of an inspection apparatus for efficiently inspecting for defects of a reticle (photomask) used for manufacture of a large-area LCD in a short time is also urgently necessary.
Mainly a transmitting optical system is used as an optical system of a defect inspection apparatus of reticle and the like. That is, a sample surface is shone using Koehler illumination as shown in FIG. 5A or critical illumination as shown in FIG. 5B and then, a transmitted light thereof is condensed and led to a detection system before image data is extracted. A defect inspection apparatus of a method using a transmitted light is introduced, for example, in JJPA, Vol. 33 (1994), pp 7156-7162, “Mask defect inspection method by database comparison with 0.25-0.35 μm sensitivity”.
In recent years, however, attempts to inspect for defects that are difficult to detect by a transmitted light by using a reflected image have been made. For example, a pattern (defect) inspection apparatus that tries to improve detection sensitivity by using an optical system as shown in FIG. 6 and mounting a transmitted/reflected light optical system is already in practical use (for example, Photomask and X-Ray Mask Technology IV, Vol. 3096 (1997), pp 404-414, “Performance of cell-shift defect inspection technique”). In such an apparatus, two wavelengths, one (λ2 in FIG. 6) used for transmitted light inspection and one (λ1 in FIG. 6) used for reflected light inspection, are separated by a filter inside a configured optical system based on wavelengths and each light is brought into a transmission sensor or a reflection sensor for detection.
Indeed, it has become necessary to make the wavelengths shorter to improve defect detection sensitivity. Further, making inspection wavelengths shorter has become necessary all the more because inspected matter increasingly requires inspection at wavelengths adjusting to those used for lithography in order to improve detection sensibility. On the other hand, making inspection wavelengths shorter makes design of an optical lens more difficult, particularly design of a lens whose aberration is made smaller for both two wavelengths. Thus, a problem arises that it is difficult for a detection apparatus that detects defects of the size of 10 nm or so to adopt an optical system in which a different wavelength is used for transmission and reflection. Therefore, the necessity of an inspection method that acquires transmission and reflection images using a single wavelength arises.
Here, when an observation is made using a transmitted light and a reflected light of a single wavelength, a method by which the same position is coaxially shone to gather observation images has generally been used (for example, U.S. Pat. No. 5,572,598; U.S. Pat. No. 5,563,702). In this method, a beam scan technology is generally adopted. FIG. 7 shows a beam scan type optical system. Since resolution can be increased for the beam scan type as beam spots formed on a reticle pattern surface become smaller, an illuminating optical system is produced by pursuing an aberration to the limit. And an inspection light is introduced from a patterned surface of a reticle to avoid an influence of thickness of the reticle. On the other hand, a light transmitted through or reflected by a reticle only needs to enter a photodiode or photomultiplier because it is necessary only to measure the amount of light. Therefore, an optical system receiving light need not pursue an aberration and thus, no particular problem arises even if measurement is made on the glass surface side.
Indeed, when realizing a simultaneous inspection of transmission and reflection in a projecting optical system in which a reticle image is formed on a sensor, in contrast to the beam scan type, resolution is determined by performance of an image-forming optical system after being transmitted through or reflected by a reticle. Here, the image-forming optical system must be arranged on the side of the pattern surface of a reticle so that the image-forming optical system is not affected by the glass thickness of a reticle. Therefore, a transmitted illumination light must be introduced from the glass surface side of a reticle and a reflected illumination light from the pattern surface side of the reticle.
To realize a simultaneous inspection of transmission and reflection in a projecting optical system under such conditions, two optical systems shown in FIG. 8 and FIG. 9 can be considered. In a method shown in FIG. 8, directions of polarized lights incident on the reticle surface after transmission and reflection are caused to be perpendicular to each other, and a light ray transmitted through the reticle and a light reflected by the reticle are separated by a polarization beam splitter. This method has an advantage of being able to image the same position on the reticle simultaneously, but due to separation of polarized light, both lights mix together to the extent that the polarized lights are disturbed by reflection by optical elements or reticle surface or the like, leading to a lower contrast. Therefore, this method may cause a problem in a reticle defect inspection apparatus that requires high-precision inspection. A method shown in FIG. 9, on the other hand, is a method by which a transmitted illumination area and a reflected illumination area are positionally separated (for example, JP-A 2004-301751(KOKAI)). By separating both areas, a transmitted light and a reflected light can be prevented from being mixed together.